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Move CRC code to a separate file.

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This commit is contained in:
Oliver Jowett 2015-01-19 23:41:26 +00:00
parent 721412dc9b
commit 07df054590
5 changed files with 293 additions and 298 deletions
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8
Makefile
View file

@ -21,11 +21,11 @@ all: dump1090 view1090
%.o: %.c dump1090.h
$(CC) $(CPPFLAGS) $(CFLAGS) $(EXTRACFLAGS) -c $<
dump1090: dump1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o
$(CC) -g -o dump1090 dump1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o $(LIBS) $(LIBS_RTL) $(LDFLAGS)
dump1090: dump1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o
$(CC) -g -o dump1090 $^ $(LIBS) $(LIBS_RTL) $(LDFLAGS)
view1090: view1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o
$(CC) -g -o view1090 view1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o $(LIBS) $(LDFLAGS)
view1090: view1090.o anet.o interactive.o mode_ac.o mode_s.o net_io.o crc.o
$(CC) -g -o view1090 $^ $(LIBS) $(LDFLAGS)
clean:
rm -f *.o dump1090 view1090

245
crc.c Normal file
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@ -0,0 +1,245 @@
// dump1090, a Mode S messages decoder for RTLSDR devices.
//
// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
#include "dump1090.h"
// Parity table for MODE S Messages.
// The table contains 112 elements, every element corresponds to a bit set
// in the message, starting from the first bit of actual data after the
// preamble.
//
// For messages of 112 bit, the whole table is used.
// For messages of 56 bits only the last 56 elements are used.
//
// The algorithm is as simple as xoring all the elements in this table
// for which the corresponding bit on the message is set to 1.
//
// The latest 24 elements in this table are set to 0 as the checksum at the
// end of the message should not affect the computation.
//
// Note: this function can be used with DF11 and DF17, other modes have
// the CRC xored with the sender address as they are reply to interrogations,
// but a casual listener can't split the address from the checksum.
//
uint32_t modes_checksum_table[112] = {
0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
};
uint32_t modesChecksum(unsigned char *msg, int bits) {
uint32_t crc = 0;
uint32_t rem = 0;
int offset = (bits == 112) ? 0 : (112-56);
uint8_t theByte = *msg;
uint32_t * pCRCTable = &modes_checksum_table[offset];
int j;
// We don't really need to include the checksum itself
bits -= 24;
for(j = 0; j < bits; j++) {
if ((j & 7) == 0)
theByte = *msg++;
// If bit is set, xor with corresponding table entry.
if (theByte & 0x80) {crc ^= *pCRCTable;}
pCRCTable++;
theByte = theByte << 1;
}
rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum
return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome.
}
//=========================================================================
//
// Code for introducing a less CPU-intensive method of correcting
// single bit errors.
//
// Makes use of the fact that the crc checksum is linear with respect to
// the bitwise xor operation, i.e.
// crc(m^e) = (crc(m)^crc(e)
// where m and e are the message resp. error bit vectors.
//
// Call crc(e) the syndrome.
//
// The code below works by precomputing a table of (crc(e), e) for all
// possible error vectors e (here only single bit and double bit errors),
// search for the syndrome in the table, and correct the then known error.
// The error vector e is represented by one or two bit positions that are
// changed. If a second bit position is not used, it is -1.
//
// Run-time is binary search in a sorted table, plus some constant overhead,
// instead of running through all possible bit positions (resp. pairs of
// bit positions).
//
struct errorinfo {
uint32_t syndrome; // CRC syndrome
int bits; // Number of bit positions to fix
int pos[MODES_MAX_BITERRORS]; // Bit positions corrected by this syndrome
};
#define NERRORINFO \
(MODES_LONG_MSG_BITS+MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2)
struct errorinfo bitErrorTable[NERRORINFO];
// Compare function as needed for stdlib's qsort and bsearch functions
static int cmpErrorInfo(const void *p0, const void *p1) {
struct errorinfo *e0 = (struct errorinfo*)p0;
struct errorinfo *e1 = (struct errorinfo*)p1;
if (e0->syndrome == e1->syndrome) {
return 0;
} else if (e0->syndrome < e1->syndrome) {
return -1;
} else {
return 1;
}
}
//
//=========================================================================
//
// Compute the table of all syndromes for 1-bit and 2-bit error vectors
void modesInitErrorInfo() {
unsigned char msg[MODES_LONG_MSG_BYTES];
int i, j, n;
uint32_t crc;
n = 0;
memset(bitErrorTable, 0, sizeof(bitErrorTable));
memset(msg, 0, MODES_LONG_MSG_BYTES);
// Add all possible single and double bit errors
// don't include errors in first 5 bits (DF type)
for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
int bytepos0 = (i >> 3);
int mask0 = 1 << (7 - (i & 7));
msg[bytepos0] ^= mask0; // create error0
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
bitErrorTable[n].syndrome = crc; // single bit error case
bitErrorTable[n].bits = 1;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = -1;
n += 1;
if (Modes.nfix_crc > 1) {
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
int bytepos1 = (j >> 3);
int mask1 = 1 << (7 - (j & 7));
msg[bytepos1] ^= mask1; // create error1
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
if (n >= NERRORINFO) {
//fprintf(stderr, "Internal error, too many entries, fix NERRORINFO\n");
break;
}
bitErrorTable[n].syndrome = crc; // two bit error case
bitErrorTable[n].bits = 2;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = j;
n += 1;
msg[bytepos1] ^= mask1; // revert error1
}
}
msg[bytepos0] ^= mask0; // revert error0
}
qsort(bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo);
// Test code: report if any syndrome appears at least twice. In this
// case the correction cannot be done without ambiguity.
// Tried it, does not happen for 1- and 2-bit errors.
/*
for (i = 1; i < NERRORINFO; i++) {
if (bitErrorTable[i-1].syndrome == bitErrorTable[i].syndrome) {
fprintf(stderr, "modesInitErrorInfo: Collision for syndrome %06x\n",
(int)bitErrorTable[i].syndrome);
}
}
for (i = 0; i < NERRORINFO; i++) {
printf("syndrome %06x bit0 %3d bit1 %3d\n",
bitErrorTable[i].syndrome,
bitErrorTable[i].pos0, bitErrorTable[i].pos1);
}
*/
}
//
//=========================================================================
//
// Search for syndrome in table and if an entry is found, flip the necessary
// bits. Make sure the indices fit into the array
// Additional parameter: fix only less than maxcorrected bits, and record
// fixed bit positions in corrected[]. This array can be NULL, otherwise
// must be of length at least maxcorrected.
// Return number of fixed bits.
//
int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits) {
struct errorinfo *pei;
struct errorinfo ei;
int bitpos, offset, res, i;
memset(&ei, 0, sizeof(struct errorinfo));
ei.syndrome = modesChecksum(msg, bits);
pei = bsearch(&ei, bitErrorTable, NERRORINFO,
sizeof(struct errorinfo), cmpErrorInfo);
if (pei == NULL) {
return 0; // No syndrome found
}
// Check if the syndrome fixes more bits than we allow
if (maxfix < pei->bits) {
return 0;
}
// Check that all bit positions lie inside the message length
offset = MODES_LONG_MSG_BITS-bits;
for (i = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
if ((bitpos < 0) || (bitpos >= bits)) {
return 0;
}
}
// Fix the bits
for (i = res = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
msg[bitpos >> 3] ^= (1 << (7 - (bitpos & 7)));
if (fixedbits) {
fixedbits[res++] = bitpos;
}
}
return res;
}

40
crc.h Normal file
View file

@ -0,0 +1,40 @@
// dump1090, a Mode S messages decoder for RTLSDR devices.
//
// Copyright (C) 2012 by Salvatore Sanfilippo <antirez@gmail.com>
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
// * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//
// * Redistributions in binary form must reproduce the above copyright
// notice, this list of conditions and the following disclaimer in the
// documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
// "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
// LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
// HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
// LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
// DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
// THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
// (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
// OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
//
#ifndef DUMP1090_CRC_H
#define DUMP1090_CRC_H
#include <stdint.h>
void modesInitErrorInfo();
uint32_t modesChecksum(unsigned char *msg, int bits);
int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits);
#endif

8
dump1090.h
View file

@ -65,14 +65,14 @@
#include <sys/ioctl.h>
#include <time.h>
#include <limits.h>
#include "rtl-sdr.h"
#include "anet.h"
#else
#include "winstubs.h" //Put everything Windows specific in here
#include "rtl-sdr.h"
#include "anet.h"
#endif
#include "rtl-sdr.h"
#include "anet.h"
#include "crc.h"
// ============================= #defines ===============================
//
// If you have a valid coaa.h, these values will come from it. If not,

290
mode_s.c
View file

@ -32,65 +32,9 @@
//
// ===================== Mode S detection and decoding ===================
//
// Parity table for MODE S Messages.
// The table contains 112 elements, every element corresponds to a bit set
// in the message, starting from the first bit of actual data after the
// preamble.
//
// For messages of 112 bit, the whole table is used.
// For messages of 56 bits only the last 56 elements are used.
//
// The algorithm is as simple as xoring all the elements in this table
// for which the corresponding bit on the message is set to 1.
//
// The latest 24 elements in this table are set to 0 as the checksum at the
// end of the message should not affect the computation.
//
// Note: this function can be used with DF11 and DF17, other modes have
// the CRC xored with the sender address as they are reply to interrogations,
// but a casual listener can't split the address from the checksum.
//
uint32_t modes_checksum_table[112] = {
0x3935ea, 0x1c9af5, 0xf1b77e, 0x78dbbf, 0xc397db, 0x9e31e9, 0xb0e2f0, 0x587178,
0x2c38bc, 0x161c5e, 0x0b0e2f, 0xfa7d13, 0x82c48d, 0xbe9842, 0x5f4c21, 0xd05c14,
0x682e0a, 0x341705, 0xe5f186, 0x72f8c3, 0xc68665, 0x9cb936, 0x4e5c9b, 0xd8d449,
0x939020, 0x49c810, 0x24e408, 0x127204, 0x093902, 0x049c81, 0xfdb444, 0x7eda22,
0x3f6d11, 0xe04c8c, 0x702646, 0x381323, 0xe3f395, 0x8e03ce, 0x4701e7, 0xdc7af7,
0x91c77f, 0xb719bb, 0xa476d9, 0xadc168, 0x56e0b4, 0x2b705a, 0x15b82d, 0xf52612,
0x7a9309, 0xc2b380, 0x6159c0, 0x30ace0, 0x185670, 0x0c2b38, 0x06159c, 0x030ace,
0x018567, 0xff38b7, 0x80665f, 0xbfc92b, 0xa01e91, 0xaff54c, 0x57faa6, 0x2bfd53,
0xea04ad, 0x8af852, 0x457c29, 0xdd4410, 0x6ea208, 0x375104, 0x1ba882, 0x0dd441,
0xf91024, 0x7c8812, 0x3e4409, 0xe0d800, 0x706c00, 0x383600, 0x1c1b00, 0x0e0d80,
0x0706c0, 0x038360, 0x01c1b0, 0x00e0d8, 0x00706c, 0x003836, 0x001c1b, 0xfff409,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000,
0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000, 0x000000
};
uint32_t modesChecksum(unsigned char *msg, int bits) {
uint32_t crc = 0;
uint32_t rem = 0;
int offset = (bits == 112) ? 0 : (112-56);
uint8_t theByte = *msg;
uint32_t * pCRCTable = &modes_checksum_table[offset];
int j;
// We don't really need to include the checksum itself
bits -= 24;
for(j = 0; j < bits; j++) {
if ((j & 7) == 0)
theByte = *msg++;
// If bit is set, xor with corresponding table entry.
if (theByte & 0x80) {crc ^= *pCRCTable;}
pCRCTable++;
theByte = theByte << 1;
}
rem = (msg[0] << 16) | (msg[1] << 8) | msg[2]; // message checksum
return ((crc ^ rem) & 0x00FFFFFF); // 24 bit checksum syndrome.
}
//
//=========================================================================
//
// Given the Downlink Format (DF) of the message, return the message length in bits.
@ -102,241 +46,7 @@ uint32_t modesChecksum(unsigned char *msg, int bits) {
int modesMessageLenByType(int type) {
return (type & 0x10) ? MODES_LONG_MSG_BITS : MODES_SHORT_MSG_BITS ;
}
//
//=========================================================================
//
// Try to fix single bit errors using the checksum. On success modifies
// the original buffer with the fixed version, and returns the position
// of the error bit. Otherwise if fixing failed -1 is returned.
/*
int fixSingleBitErrors(unsigned char *msg, int bits) {
int j;
unsigned char aux[MODES_LONG_MSG_BYTES];
memcpy(aux, msg, bits/8);
// Do not attempt to error correct Bits 0-4. These contain the DF, and must
// be correct because we can only error correct DF17
for (j = 5; j < bits; j++) {
int byte = j/8;
int bitmask = 1 << (7 - (j & 7));
aux[byte] ^= bitmask; // Flip j-th bit
if (0 == modesChecksum(aux, bits)) {
// The error is fixed. Overwrite the original buffer with the
// corrected sequence, and returns the error bit position
msg[byte] = aux[byte];
return (j);
}
aux[byte] ^= bitmask; // Flip j-th bit back again
}
return (-1);
}
*/
//=========================================================================
//
// Similar to fixSingleBitErrors() but try every possible two bit combination.
// This is very slow and should be tried only against DF17 messages that
// don't pass the checksum, and only in Aggressive Mode.
/*
int fixTwoBitsErrors(unsigned char *msg, int bits) {
int j, i;
unsigned char aux[MODES_LONG_MSG_BYTES];
memcpy(aux, msg, bits/8);
// Do not attempt to error correct Bits 0-4. These contain the DF, and must
// be correct because we can only error correct DF17
for (j = 5; j < bits; j++) {
int byte1 = j/8;
int bitmask1 = 1 << (7 - (j & 7));
aux[byte1] ^= bitmask1; // Flip j-th bit
// Don't check the same pairs multiple times, so i starts from j+1
for (i = j+1; i < bits; i++) {
int byte2 = i/8;
int bitmask2 = 1 << (7 - (i & 7));
aux[byte2] ^= bitmask2; // Flip i-th bit
if (0 == modesChecksum(aux, bits)) {
// The error is fixed. Overwrite the original buffer with
// the corrected sequence, and returns the error bit position
msg[byte1] = aux[byte1];
msg[byte2] = aux[byte2];
// We return the two bits as a 16 bit integer by shifting
// 'i' on the left. This is possible since 'i' will always
// be non-zero because i starts from j+1
return (j | (i << 8));
aux[byte2] ^= bitmask2; // Flip i-th bit back
}
aux[byte1] ^= bitmask1; // Flip j-th bit back
}
}
return (-1);
}
*/
//
//=========================================================================
//
// Code for introducing a less CPU-intensive method of correcting
// single bit errors.
//
// Makes use of the fact that the crc checksum is linear with respect to
// the bitwise xor operation, i.e.
// crc(m^e) = (crc(m)^crc(e)
// where m and e are the message resp. error bit vectors.
//
// Call crc(e) the syndrome.
//
// The code below works by precomputing a table of (crc(e), e) for all
// possible error vectors e (here only single bit and double bit errors),
// search for the syndrome in the table, and correct the then known error.
// The error vector e is represented by one or two bit positions that are
// changed. If a second bit position is not used, it is -1.
//
// Run-time is binary search in a sorted table, plus some constant overhead,
// instead of running through all possible bit positions (resp. pairs of
// bit positions).
//
struct errorinfo {
uint32_t syndrome; // CRC syndrome
int bits; // Number of bit positions to fix
int pos[MODES_MAX_BITERRORS]; // Bit positions corrected by this syndrome
};
#define NERRORINFO \
(MODES_LONG_MSG_BITS+MODES_LONG_MSG_BITS*(MODES_LONG_MSG_BITS-1)/2)
struct errorinfo bitErrorTable[NERRORINFO];
// Compare function as needed for stdlib's qsort and bsearch functions
int cmpErrorInfo(const void *p0, const void *p1) {
struct errorinfo *e0 = (struct errorinfo*)p0;
struct errorinfo *e1 = (struct errorinfo*)p1;
if (e0->syndrome == e1->syndrome) {
return 0;
} else if (e0->syndrome < e1->syndrome) {
return -1;
} else {
return 1;
}
}
//
//=========================================================================
//
// Compute the table of all syndromes for 1-bit and 2-bit error vectors
void modesInitErrorInfo() {
unsigned char msg[MODES_LONG_MSG_BYTES];
int i, j, n;
uint32_t crc;
n = 0;
memset(bitErrorTable, 0, sizeof(bitErrorTable));
memset(msg, 0, MODES_LONG_MSG_BYTES);
// Add all possible single and double bit errors
// don't include errors in first 5 bits (DF type)
for (i = 5; i < MODES_LONG_MSG_BITS; i++) {
int bytepos0 = (i >> 3);
int mask0 = 1 << (7 - (i & 7));
msg[bytepos0] ^= mask0; // create error0
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
bitErrorTable[n].syndrome = crc; // single bit error case
bitErrorTable[n].bits = 1;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = -1;
n += 1;
if (Modes.nfix_crc > 1) {
for (j = i+1; j < MODES_LONG_MSG_BITS; j++) {
int bytepos1 = (j >> 3);
int mask1 = 1 << (7 - (j & 7));
msg[bytepos1] ^= mask1; // create error1
crc = modesChecksum(msg, MODES_LONG_MSG_BITS);
if (n >= NERRORINFO) {
//fprintf(stderr, "Internal error, too many entries, fix NERRORINFO\n");
break;
}
bitErrorTable[n].syndrome = crc; // two bit error case
bitErrorTable[n].bits = 2;
bitErrorTable[n].pos[0] = i;
bitErrorTable[n].pos[1] = j;
n += 1;
msg[bytepos1] ^= mask1; // revert error1
}
}
msg[bytepos0] ^= mask0; // revert error0
}
qsort(bitErrorTable, NERRORINFO, sizeof(struct errorinfo), cmpErrorInfo);
// Test code: report if any syndrome appears at least twice. In this
// case the correction cannot be done without ambiguity.
// Tried it, does not happen for 1- and 2-bit errors.
/*
for (i = 1; i < NERRORINFO; i++) {
if (bitErrorTable[i-1].syndrome == bitErrorTable[i].syndrome) {
fprintf(stderr, "modesInitErrorInfo: Collision for syndrome %06x\n",
(int)bitErrorTable[i].syndrome);
}
}
for (i = 0; i < NERRORINFO; i++) {
printf("syndrome %06x bit0 %3d bit1 %3d\n",
bitErrorTable[i].syndrome,
bitErrorTable[i].pos0, bitErrorTable[i].pos1);
}
*/
}
//
//=========================================================================
//
// Search for syndrome in table and if an entry is found, flip the necessary
// bits. Make sure the indices fit into the array
// Additional parameter: fix only less than maxcorrected bits, and record
// fixed bit positions in corrected[]. This array can be NULL, otherwise
// must be of length at least maxcorrected.
// Return number of fixed bits.
//
int fixBitErrors(unsigned char *msg, int bits, int maxfix, char *fixedbits) {
struct errorinfo *pei;
struct errorinfo ei;
int bitpos, offset, res, i;
memset(&ei, 0, sizeof(struct errorinfo));
ei.syndrome = modesChecksum(msg, bits);
pei = bsearch(&ei, bitErrorTable, NERRORINFO,
sizeof(struct errorinfo), cmpErrorInfo);
if (pei == NULL) {
return 0; // No syndrome found
}
// Check if the syndrome fixes more bits than we allow
if (maxfix < pei->bits) {
return 0;
}
// Check that all bit positions lie inside the message length
offset = MODES_LONG_MSG_BITS-bits;
for (i = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
if ((bitpos < 0) || (bitpos >= bits)) {
return 0;
}
}
// Fix the bits
for (i = res = 0; i < pei->bits; i++) {
bitpos = pei->pos[i] - offset;
msg[bitpos >> 3] ^= (1 << (7 - (bitpos & 7)));
if (fixedbits) {
fixedbits[res++] = bitpos;
}
}
return res;
}
//
// ============================== Debugging =================================
//
// Helper function for dumpMagnitudeVector().